Carbon dioxide is taken in by plants during photosynthesis
and produced by animal respiration. The building of an organism requires
a way of making organic molecules from carbon dioxide. This is achieved
during the Calvin cycle of photosynthesis
in green plants. By means of this cycle, plants incorporate carbon dioxide
into sugars which form the basis for all of
the organic molecules synthesized by cells. Carbon dioxide is also an important
greenhouse gas.

Liquid carbon dioxide, formed under pressure, is used in fire extinguishers.
Carbon dioxide is also used to make carbonated drinks. When dissolved in
water an equilibrium is set up, with carbonate, bicarbonate,
and hydrogen ions formed, and a low concentration
of carbonic acid (H2CO3).

density (at 0°C)

1.98 g/dm3

melting point

-56.6°C

boiling point

-78.5°C

molar mass

44.0095 g/mol

Carbon dioxide and global warming

Carbon is formed by the combustion of carbon and carbon compounds (such
as fossil fuels and biomass),
by respiration, which is a slow combustion in animals and plants, and by
the gradual oxidation of organic matter in the soil.

Carbon dioxide contributes about 60% of the potential global
warming effect of manmade emissions of greenhouse gases. Although living
organisms naturally emits this gas, these emissions are offset by the uptake
of carbon dioxide by plants during photosynthesis; they therefore tend to
have no effect on atmospheric concentrations. The burning of fossil fuels,
however, releases carbon dioxide fixed by plants many millions of years
ago and thus increases its concentration in the atmosphere.

The global warming potential (GWP) of other greenhouse gases is measured
in relation to that of carbon dioxide, which by international scientific
convention is assigned a value of 1.

Carbon dioxide equivalent

Carbon dioxide equivalent is the amount of carbon dioxide by weight emitted
into the atmosphere that would produce the same estimated radiative forcing
as a given weight of another radiatively active gas. Carbon dioxide equivalents
are computed by multiplying the weight of the gas being measured (for example,
methane) by its estimated global warming potential (which is 21 for methane).
"Carbon equivalent units" are defined as carbon dioxide equivalents multiplied
by the carbon content of carbon dioxide (i.e., 12/44).

Dry
ice

Solid carbon dioxide, or dry ice, is used as a refrigerant for transporting
perishables. It is made by compressing carbon dioxide gas to a pressure
of about - 7MN/m2 at -57°C, when it liquefies; it is then
expanded adiabatically to atmospheric pressure and cools, solid carbon dioxide
"snow" separating. This is compressed into blocks. Dry ice sublimes at -78.5°C;.

Carbon dioxide as a possible solvent
for alien life

It has been suggested that on some worlds carbon dioxide, rather than water,
might serve as a suitable solvent for life.
Although this may sound unlikely, when carbon dioxide is subjected to pressures
of 90 atmospheres or more, such as are found on planets such as Venus
or Neptune, it enters a quasi-liquid, supercritical
state. In this unfamiliar form, carbon dioxide is as heavy as water but
almost as easy to move through as air because molecules of carbon dioxide
don't stick together as tightly as the molecules in most liquids. When certain
enzymes (biological catalysts) are placed
in supercritical carbon dioxide, they work as well as they do in other substances
that have been suggested as alternative solvents for life, such as hexane
and ether.